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Coloration in Acropora nana By Dana Riddle and Andy Amussen

Dana Riddle writes on his experiments with coloation in acropora, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Coloration in Acropora nana

By Dana Riddle and Andy Amussen Riddle Aquatic Laboratories

Captive coral propagation has allowed us to observe genetically identical animals under varying aquarium conditions. Specifically, we have gathered information on coloration and are beginning to understand what factors are involved to induce coral pigment production.

Corals are often brown, due to the presence of zooxanthellae (symbiotic, photosynthetic dinoflagellates). Some corals will develop color brilliant coloration. What triggers this coloration? What can hobbyists do to maintain coloration in their captive corals?

This month's study involves the Pacific stony coral, Acropora nana. This attractive coral is often found on reefs with good water movement and, presumably, intense light. We hypothesized at least three factors could be involved with coloration - light intensity, water motion and alkalinity levels. (This, of course, assumes that other conditions in the aquarium are correct, e.g., specific gravity of ~1.025; pH values of 8.0-8.4, etc.). We designed the following experiments to test our theories: Test #1: Low Light, Low Water Movement and High Alkalinity Levels, Test #2. Low Light, Higher Water Motion and Low Alkalinity Levels and Test #3: High Light, Low Water Motion and High Alkalinity Levels.

Some hobbyists may be surprised that we did not include ultraviolet radiation (UV) as a factor. We have measured UV energy in dozens of aquaria and have reached the conclusion that high doses of UV energy do not play a role in making corals turn purple, red, blue, etc. We have noted that green fluorescent pigments may be produced under conditions of higher UV energy (about 30 microwatts per square centimeter per second).

Instrumentation We used the following equipment in our studies:

Quantum Meter with submersible cosine-corrected sensor . LiCor, Lincoln, Nebraska.

UV Radiometer, with UVA sensor (maximum sensitivity @ 365nm); UVB sensor (maximum sensitivity @ 310 nm). UVP Products, Upland, California.

Electromagnetic Water Velocity Meter , Flo-Mate 2000, Marsh-McBirney, Frederick, Maryland.

Test #1: Low Light, Low Water Movement and High Alkalinity Levels

One Acropora nana fragment was maintained for 60 days in an aquarium with high alkalinity levels (up to 16 dKH, or 5.7 meq/l). Light was provided by two 110watt VHO fluorescent lamps - one daylight, one Coralife 10,000° K. Light measurements were 105 µEinsteins per square meter per second (abbreviated as µE), or about 5,000 lux. The coral remained brown. Little growth was noted. (We should mention that this coral fragment had time to heal after removal from the parent colony.)

Test #2. Low Light, Higher Water Motion and Low Alkalinity Levels

This test used dozens of fragments (genetically identical to that used in Test #1). Light levels were low (~100 µEinsteins -5,000 lux). Water motion was measured at up to 0.11 ft/second. An alkalinity level of about 7 dKH (2.5 meq/l) was maintained. These corals remained brown. (We normally recommend higher water velocities; however, these coral fragments were stick-like. Thus they required less water motion than thicket-like adult colonies.) Some growth was noted - mostly encrusting growths around the bases.

Test #3: High Light, Low Water Motion and High Alkalinity Levels

Using the same aquarium and fragment as in Test #1, we added a 175watt metal halide lamp (4,300° K and shielded for UV energy). Light intensity increased from 105 µE to 210 µE (about 10,000 lux). Alkalinity was as high as 16 dKH and as low as 8.4 dKH (5.7 and 3 meq/l, respectively). The coral's axial tips turned purple after several weeks. The time between removing the fragments from the adult colony and the beginning of Test #3 was about 3 months.

Observed Light Levels for Acropora nana

Light Level (µEinsteins)





None (Recently fragmented)



















Our short study suggests that light is the most important factor for the promotion of coloration in this Acropora species. Our observations also suggest that pigments may not become apparent for 30 days or so after light levels are increased. This may take slightly longer if fragments or cuttings are taken from a parent colony, as they seem to first direct their energies to recovery.

Does lamp spectra influence coral coloration? We tend to think not (although caveats do apply). Our contentions are these: Zooxanthellae play an important part in coral growth and, hence, pigmentation. Zooxanthellae require only a certain amount of light for the process of photosynthesis. Further, zooxanthellae require light in the blue and red portions of the spectrum (and other portions if accessory pigments are present). When light levels no longer increase the rate of photosynthesis, the process is "saturated". We believe that light saturation can occur in particular portions of the spectrum (that is, blue or red or green) with most lamps. Lamp Kelvin temperature does not matter if sufficient amounts of certain portions of the spectrum are produced. However, aesthetics play an important part in display aquaria and lamp Kelvin temperature is likely a prime consideration.

Based on our studies and observations, we believe that light intensity of at least 125-150 µE (6,250 - 7,500 lux) is required for colorful Acropora nana specimens. Four 40-watt regular output fluorescent lamps will produce this much radiation to a depth of about 6 inches in an aquarium. Two 110watt VHO fluorescent lamps will deliver a sufficient amount of radiation to a depth of 18 inches.

We wish to thank the following persons for their participation in this study: Walter Bobe, John Brandt (Seashell Pets), John Cummings, Noel Curry (Scientific Corals) and Steve Pratt

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Last modified 2006-11-23 01:39